Messages in this thread | | | Date | Thu, 12 Jan 2012 01:52:57 -0800 | From | Josh Triplett <> | Subject | Re: [PATCH] rcu: avoid checking for constant |
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On Thu, Jan 12, 2012 at 10:25:23AM +0100, Jan Engelhardt wrote: > rcu: avoid checking for constant > > When compiling kernel or module code with -O0, "offset" is no longer > considered a constant, and therefore always triggers the build error > that BUILD_BUG_ON is defined to yield. > > What is the rationale between the forced constant check, > introduced in 9ab1544eb4196ca8d05c433b2eb56f74496b1ee3?
This question shouldn't live in the commit message. Please replace it with an explanation of the change instead.
> Signed-off-by: Jan Engelhardt <jengelh@medozas.de> > --- > include/linux/rcupdate.h | 22 ++++------------------ > 1 files changed, 4 insertions(+), 18 deletions(-) > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > index 2cf4226..5e7286d 100644 > --- a/include/linux/rcupdate.h > +++ b/include/linux/rcupdate.h > @@ -795,24 +795,6 @@ static inline notrace void rcu_read_unlock_sched_notrace(void) > #define RCU_INIT_POINTER(p, v) \ > p = (typeof(*v) __force __rcu *)(v) > > -static __always_inline bool __is_kfree_rcu_offset(unsigned long offset) > -{ > - return offset < 4096; > -} > - > -static __always_inline > -void __kfree_rcu(struct rcu_head *head, unsigned long offset) > -{ > - typedef void (*rcu_callback)(struct rcu_head *); > - > - BUILD_BUG_ON(!__builtin_constant_p(offset)); > - > - /* See the kfree_rcu() header comment. */ > - BUILD_BUG_ON(!__is_kfree_rcu_offset(offset)); > - > - call_rcu(head, (rcu_callback)offset); > -} > - > /** > * kfree_rcu() - kfree an object after a grace period. > * @ptr: pointer to kfree > @@ -836,6 +818,10 @@ void __kfree_rcu(struct rcu_head *head, unsigned long offset) > * Note that the allowable offset might decrease in the future, for example, > * to allow something like kmem_cache_free_rcu(). > */ > +#define __kfree_rcu(head, offset) \ > + call_rcu(head, (void (*)(struct rcu_head *))(unsigned long)(offset) + \ > + BUILD_BUG_ON_ZERO((offset) >= 4096)) > +
I had to stare at this for a while, and look up the definition of BUILD_BUG_ON_ZERO. Naturally I assumed that BUILD_BUG_ON_ZERO(arg) meant BUILT_BUG_ON((arg) == 0), which would have made the logic backwards here. However, per the definition it just provides a zero-returning version of BUILD_BUG_ON. Ow.
In any case, __kfree_rcu has void return type, so how about just using do { ... } while(0) and BUILD_BUG_ON instead? That seems significantly clearer.
Apart from that, I can live with this, though it seems horribly backwards to replace an inline with a macro rather than the other way around.
- Josh Triplett
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